.header {
  width: 1000px;
}
@media screen and (min-width: 768px) {
  .header {
    height: 100px;
    width: 500px;
  }
}

.footer {
  color: blue;
}
@media screen and (min-width: 768px) {
  .footer {
    height: 200px;
    color: red;
  }
}